EDI vs Mixed Bed DI: Industrial Selection, Sizing & OPEX

september 23, 2025

This practical guide compares edi vs mixed bed polishing in an industrial di water system. You’ll get a decision matrix, field-tested sizing rules, typical OPEX differences, feed-water windows, polishing steps, and a concise BOM you can send to procurement. Focus: commercial/industrial users — electronics, laboratories, food & beverage, hotels, plating, and light pharma (note: DI is not WFI).

After RO, choose EDI for continuous operation without bulk chemicals, or mixed bed for tolerance and simple retrofits.

1) Basics: What They Do

In most industrial trains, RO removes bulk salts first. A polisher then drives ionic content to the final target: EDI (electrodeionization) uses ion-exchange resins and membranes under an electric field for continuous polishing without bulk acid/caustic handling; a mixed bed uses separate cation/anion resins in one vessel that you regenerate with acid and caustic between runs.

Quality is specified as resistivity (MΩ·cm) of conductivity (μS/cm). Quick conversion: conductivity (μS/cm) = 1 / resistivity (MΩ·cm). Many plants target 1–10 MΩ·cm; Type I lab water requires ~18 MΩ·cm with a polished loop.

Selection flow: feed stability, chemical policy, uptime and staffing often decide edi vs mixed bed.

2) EDI vs Mixed Bed — Decision Matrix

Criteria	EDI polishing	Mixed bed DI
Operating mode	Continuous, automated; no bulk chemicals	Batch; periodic acid/caustic regeneration and neutralization
Feed-water window	Best with stable RO permeate; low hardness/CO₂/silica; low TOC	More tolerant to swings; handles intermittent operation well
Quality stability	Very stable resistivity at constant load	High resistivity achievable; may drift as resin exhausts
Uptime & staffing	High uptime; commissioning requires expertise	Simpler hardware; regeneration skills and safety training needed
OPEX	Electrical power; occasional CIP	Acid/caustic, rinse water, neutralization, PPE & handling
Voetafdruk	Compact skid; power supply & controls	Vessels + chemical storage + neutralization system
When to prefer	Continuous production, limited chemical handling, tight specs	Retrofits, frequent stops/starts, wide feed variability, lower capex

Rule of thumb: if your site restricts bulk chemicals or aims for continuous lights-out operation, pick EDI. If you need tolerance to irregular feeds, stop–start shifts, or want the lowest upfront cost, start with a mixed bed.

3) Sizing Rules & Quick Formulas

3.1 EDI Sizing

Confirm RO permeate meets vendor window (low hardness; low CO₂, silica and TOC; temperature factor applied).
Select stacks by product flow (m³/h) and expected ionic load; request the vendor’s sizing sheet and add a seasonal safety margin.
Provide bypass/recirculation and measure resistivity en flow at the skid outlet; alarm on deviations.

3.2 Mixed Bed Capacity (rule-of-thumb)

Express ionic load after RO as meq/L. Estimate resin volume for a target run time:

Resin volume (L) ≈ [meq/L × product flow (m³/h) × run time (h)] ÷ [resin capacity (eq/L)] × safety factor

Choose run time (e.g., 8–24 h) to align with shifts. Use the resin supplier’s capacity (depends on grade and regeneration regime).

3.3 Regeneration Chemical Estimate (general form)

Define required equivalents to restore the bed (from vendor). A general stoichiometric form is:

Acid volume (L)  ≈  required eq ÷ acid normality (N) ÷ efficiency
Caustic volume (L) ≈ required eq ÷ caustic normality (N) ÷ efficiency

“Efficiency” reflects practical overfeed and contact losses; confirm with your resin datasheet and SOP before ordering chemicals.

3.4 Quality Conversion (Resistivity ↔ Conductivity)

Resistivity (MΩ·cm) Conductivity (μS/cm)

Tip: conductivity (μS/cm) = 1 / resistivity (MΩ·cm).

4) OPEX, Footprint & Operations

EDI OPEX: primarily electrical power; watch temperature/CO₂–driven ionic load swings; run vendor-recommended CIP as needed.
Mixed Bed OPEX: acid/caustic consumption, rinse water, neutralization system operation, PPE and handling/safety training.
Downtime & variability: EDI prefers continuous duty; mixed beds tolerate stop–start operation and load spikes better.
Compliance & safety: chemical storage with secondary containment, ventilation, eyewash/shower, and documented regeneration/neutralization SOPs.

At steady load with chemical restrictions, EDI tends to win; under variable feeds or frequent stops, mixed bed is simpler.

5) Polishing, Storage & Distribution

UV (254 nm) for microbial control; add 185 nm UV for TOC if SOP requires.
0.2 μm final filter before points of use or loop return.
Use a stainless-steel storage tank with vent filter and spray ball; keep loop velocity high enough to maintain quality.

6) Materials & Instrumentation

Pipes: UPVC/PP for pretreatment; SS316L or sanitary plastics downstream of polishing.
Key meters: resistivity/conductivity (RO permeate, polisher outlet, loop return), TOC (if required), SDI, flow, and ΔP.
Controls: PLC/HMI with interlocks on quality deviations and alarms; data logging for audits.

7) BOM Template & Related Products

Pretreatment: MMF, GAC, softener or antiscalant dosing, cartridge filters
RO skid: high-pressure pump, vessels/elements, instrumentation, CIP ports
Polisher: EDI stacks of mixed bed resin vessel with regeneration/CIP kit
Polishing: UV 254 (option 185), 0.2 μm final filter
Storage/loop: SS316L tank, recirculation pump, resistivity at outlet/return, TOC if needed
Docs: P&ID, layout, FAT/SAT checklists, IQ/OQ templates

8) FAQ

Is EDI always better than a mixed bed?

No. EDI is ideal when chemicals are restricted and operation is continuous. Mixed beds are cheaper to start and tolerate feed swings and stop–start duty.

How do I know my RO permeate is good enough for EDI?

Check hardness, CO₂, silica and TOC against your vendor’s window. Stable temperature and load improve EDI performance and power efficiency.

What resistivity should I target after polishing?

Follow your SOP or industry guidance: many plants target 1–10 MΩ·cm; Type I lab water requires ~18 MΩ·cm and a polished loop.

Can I retrofit EDI to replace my mixed bed?

Yes — validate RO permeate window and utilities first, then size EDI stacks. Many sites keep a small mixed bed as emergency backup.

9) References (External)

Stark Water Engineering Team — Field experience in RO, EDI and mixed-bed DI for electronics, labs, F&B and hospitality.

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